If you have seen my post in another thread about semidiesels, I asked if the term "Semidiesel" was not a non Lanz invention.
Since posting I have had a look through some old publicity brochures for the Société Française Vierzon hot bulb tractor 201.

The description is a "Semi Diesel, 2 stroke, scavenging by the sump, single cylinder horizontal. Compression Ratio 6:1.

So in fact this engine is very very similar in construction, has a hot bulb and trembler spark ignition as my 5506 Lanz has.

Even though I'm not the "greater authority" you are looking for, I'd like to say something about the Hot-Bulb or Semi-Diesel Engines.

As Theo said in the other thread, many people aren't aware of the fact, that the hot-bulb engine isn't a diesel engine.

Alex wrote:" ...the hot bulb engine is definitively no diesel engine." And he gave the explanation: "The hot bulb engine ignites the fuel by the glowing walls of the hot bulb and not by compression temperature." In addition he wrote: "It is more or less a mixture of diesel and gasoline engine". In my humble opinion it is exactly that, too: a hybrid of diesel and gasoline engine. Alex mentioned similarities to a gasoline engine like the low compression ratio. He also mentioned similarities to the diesel engine, like the internal fuel-mixture generation for instance. So you could say it is more or less only half a diesel engine. And so many called it "semi-diesel" as you read in the brochure for the Société Française Vierzon hot bulb tractor.

In Sweden, many hot-bulb engines were manufactured and sold. The "National Machinery Testing Institute in Sweden (SMP)" tested several hot-bulb engines over the years. I've read an interesting SAE-Paper from the year 2002 called "Early Swedish Hot-Bulb Engines - Efficiency and Performance Compared to Contemporary Gasoline and Diesel Engines" in which the author (Olof Erlandsson) compares the SMP-tests and describes the similarities between Hot-Bulb Engines and "Homogenous Charge Compression Ignition" (HCCI) Engines. He doesn't even mention the company name LANZ but the name "semi-diesel" for Hot-Bulb Engine in general.

Let's turn our focus to the LANZ-Engines. I don't know if LANZ called the new engines "Halbdiesel-Motor", like the "New Lanz Engine of 17 HP" for instance. All I know is, that they still called it "Glühkopfmotor". If you refer to the manual on Alex' site: 17 and 22 HP manual you will find, that it is called "Mitteldruck-Glühkopfmotor". I also read a MTZ-Paper from the year 1953 called "Neue Wege im Glühkopfmotorenbau - Der neue Glühkopfmotor der Heinrich-Lanz AG., Mannheim" in which the new Lanz Engines with 17,22 and 28 HP are described as hot-bulb engines (and tested). The user "bulldogfan" gave the explanation in the other thread why it is still called hot-bulb engine : "What is commonly known as 'Halbdiesel' has a hot bulb too, the only difference is that it's smaller and you can't blow torch it. So please explain as to where the difference between a hotbulb and a halbdiesel is if they both have a hotbulb?". In my opinion it is the same with the last LANZ engines, the so called "Volldiesel" or full-diesel engines. I have to mention that as far as I know the compression ratio is a bit higher at the "Volldiesel" engines (12:1) than at the "Halbdiesel" engines (11:1). But they both still have a small compression ratio and a nearly uncooled combustion chamber. Of course, the engine design between the "real" hot-bulb engines and the later LANZ engines changed dramatically, Alex mentioned the differences like the missing prechamber or injection-timing and -pressure for instance. The changes in engine design helped to increase the efficiency - the engines got better and had a very low fuel consumption - but they still remain hot-bulb engines because the compression isn't high enough to ignite the air-fuel mixture and because of the externally-supplied ignition with the help of the hot walls of the combustion chamber.

You can refer to a text about the LANZ hot-bulb engines in which also the last LANZ-Engines are mentioned. The author doesn't use the terms "Halbdiesel" or "Volldiesel" and thus draws no distinction between them. He also calls the old hot-bulb engines "Mitteldruckmotor" or as you wrote "medium compression" engine and gives the opinion, that you could "already" call the last LANZ engines "Zweitakt-Dieselmotor" (2-stroke-diesel engine) - but all of them are hot-bulb engines. Unfortunately it is written in German.

But I also found a French Lanz-brochure from the D2416 in which LANZ (the Company, not Heinrich himself of course) describes the combustion process: 24 HP manual (french). The fuel still isn't ignited only by compression temperature but too by the hot walls of the combustion chamber. To give you an excerpt: "Le mélange combustible-air s'enflamme au contact de la téte d'allumage chaude, l'explosion repousse le piston vers l'arrière, celui-ci comprime l'air dans le carter de vilebrequin." I have to mention, that the combustion isn't an explosion, but that is written on another page.

In the last production period the official description of the engine was changed into "Zweitakt-Dieselmotor" and the tractors were named "LANZ Diesel". I think some guy in the marketing-departement thought something like: "We have to show the farmers that we now sell modern ( and that means DIESEL - and green paint,of course ) tractors. Don't you think you could call these hot-bulb, semi-diesel or whatever tractors DIESEL, too?" In my opinion LANZ respectively John-Deere had recognized, that the Diesel engine had won the race and changed first of all the description and then the combustion process by manufacturing multicylinder (real) Diesel-tractors.

I don't know why everybody calls the "new" LANZ engine "Halbdiesel" or semi-diesel and the last LANZ engines "Volldiesel" or full-diesel. In my opinion the combustion process is still the same. But I remember when I first heard of these strange engines and that they were started with gasoline and then switched to diesel fuel. I thougt: "Ok, 'Halbdiesel' means starting with gasoline and then firing with diesel fuel and 'Volldiesel' means it is a real diesel engine because it is started with the glow-plug!". Perhaps something like that is the reason for the wide-spread usage of these terms today?

The original literature I know, be it sales material or technical explanations written in English, French and Dutch never mention the term 'hotbulb'. The 'Glühkopf' engines were called 'Semi- Diesel' and the 'Halb/Volldiesel' were called (Lanz)Diesel. I don't know much about the 17/22 HP engines, but the 28 HP was also called Diesel, even in German. The really early material called the 'Glühkopf' Crude oil engine, Moteur à huile lourde (confusingly, huile lourde is still written on post war 'Glühkopf' machines). I don't know what they were called in Dutch. As far as I can tell the non German literature is consistent.

I don't think that the design of the cylinderhead, fuel injection pressure and timing have any bearing on what type of (Diesel) engine it is.
There are also many other Diesel engines that have spark plugs and sometimes carburettors to ease the starting procedure on petrol, but remain standard fullcompression Diesel engines.

I haven never tried it, but if it were possible (which i'm sure it is) to start a 'Glühkopf' on diesel with a good glowplug and a normal starter, would you call it a 'full diesel'?

just to add a little more confusion to the story, the guy who invented the hotbulb engine also invented the diesel engine (and forgot to patent it), but realised that it would be to difficult to manufacture an hence the need for a simpler injection system which required other means of dissipating fuel (diesel) for the compression to be able to ignite it, and that's where the hotbulb comes in. i really don't see what a hotbulb engine has in common with a petrol engine.